Noise control is needed to make the system unobtrusive – noisy systems get turned down or off. Include proper silencers on supply and exhaust – these are big (900 mm long and 100 mm larger diameter than the ductwork) so if possible fit them between ceiling joists. Branched systems may also require cross-talk attenuators on the supply to prevent noise transmission between rooms via the duct.
The Passivhaus ventilation protocol worksheet gives design figures of 25 dB(A) in living and bedrooms and 35 dB(A) in the room containing the MVHR fan unit. These figures are used to calculate attenuation required – they are too low to accurately measure, at least during the day on a busy building site. Once the system is running you shouldn’t be able to hear the ventilation in bedrooms or living rooms.
Room terminals are different for supply and extract, and there are different terminals for wall supply, directional ceiling supply, and all-round ceiling supply. The position in the room depends on the terminal type: directional terminals are often used to throw air in from the doorway; standard terminals need to be near the middle of the room (but avoid locating over beds) – in front of windows is usually good as the terminals are then unlikely to be obstructed by high furniture. Extract terminals can be put on a wall or ceiling; always put these on the far side of the room from the door. In housing, door undercuts are the normal way to allow air to flow out of supply rooms and into extract rooms. The minimum undercut is 10mm above final floor finish.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015.